Russia will quickly start losing a conventional war. They would turn to nukes. When it seems like Russia is losing, all nuclear nations would be on high alert. They being on high alert with fingers on triggers is bad not just for Nato and Russia but also for non participating countries like China, India, Pakistan, Israel, NK because when there is a nuclear war on the horizon, everyone wants to be on the alert. However, each country raising alert will cause suspicion on the other country. For example, China raising its alert level will trigger India to do the same followed by Pakistan. All it takes now is one nuke to fly (may be between Nato and Russia or even a false launch detection). Now all countries will trigger all their nukes purely out of suspicion in the fog of war due to panic.
So that's why Russia can shoot/burn or destroy people /cities /infrastructure, NATO will not lift a single finger in terms of actual participation in war to help Ukraine (whatever the public opinion might be).